Well, the way I use a device with a monitor is a different kind of UX though. Usually it involves a keyboard and mouse, not a touch screen, and I can do certain things better through that kind of input. For that reason I personally like having different devices that are specific to different use cases, as I can express myself better then through the mechanical intuition that I have built up, instead of shoehorning one generic way to do everything. Phones I mostly use to quickly check things such as news, sometimes to watch videos, and messaging/calling, I don't need a large screen for that because at some point it compromises my ability to hold and operate the phone with one hand. Having a virtual environment that responds to your body/hands of course allows you to emulate all types of things, when it gets to that point I think I agree that having one device is the future.

Bad eye sight is a good one, but I don't think it explains the popularity of large screens fully, are there really so many with bad eye sight to the point that they need a larger screen?